Cells of this type, which are described in DE 30 22 818, are characterised by a favourable viewing-angle dependence of the contrast and have become predominant very generally, in particular in high-information displays addressed by means of an active matrix.
However, it is disadvantageous in such cells that an effect known as inverse contrast is observed when grey shades are displayed; this means that the contrast, after increasing initially when a relatively high voltage is applied, first drops again. FIG. 1 shows electro-optical characteristic lines at various viewing angles .theta. for a TN cell which has a twist angle of 90.degree. and a cell separation of d=5.94 .mu.m, and contains the following liquid-crystal mixture LC I:
5.0% of 4-(trans-4-pentylcyclohexyl)-1-fluorobenzene PA0 5.0% of 4-(trans-4-heptylcyclohexyl)-1-fluorobenzene PA0 8.0% of 4-[trans-4-(trans-4-ethylcyclohexyl)cyclohexyl]-1-trifluoromethoxybenzene PA0 9.0% of 4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]-1-trifluoromethoxybenzene PA0 9.0% of 4-[trans-4-(trans-4-pentylcyclohexyl)cyclohexyl]-1-trifluoromethoxybenzene PA0 11.0% of 1-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]-2-(3,4-difluorophenyl) ethane PA0 11.0% of 1-[trans-4-(trans-4-pentylcyclohexyl)cyclohexyl]-2-(3,4-difluorophenyl)eth ane PA0 5.0% of 2,6-difluoro-4-(trans-4-propylcyclohexyl)-3',4'-difluorobiphenyl PA0 4.0% of 2,6-difluoro-4-(trans-4-pentylcyclohexyl)-3',4'-difluorobiphenyl PA0 8.0% of 4-[trans-4-(trans-4-propylcyclohexyl)cyclohexyl]-2,6-difluoro-1-difluorome thoxybenzene PA0 14.0% of 4-[trans-4-(trans-4-pentylcyclohexyl)cyclohexyl]-2,6-difluoro-1-difluorome thoxybenzene PA0 6.0% of 4-trifluoromethoxyphenyl trans-4-(trans-4-propylcyclohexyl)cyclohexylcarboxylate PA0 6.0% of 4-trifluoromethoxyphenyl trans-4-(trans-4-pentylcyclohexyl)cyclohexylcarboxylate
The liquid-crystal mixture LC I has the following properties:
______________________________________ Clearing point N 103.degree. C. I Viscosity .eta. (20.degree. C.) = 21 mm.sup.2 s.sup.-1 Dielectric anisotropy .DELTA..epsilon. (20.degree. C., 1 kHz) = 6.9 Optical anisotropy .DELTA.n (20.degree. C., 589 nm) = 0.0848 ______________________________________
The optical path length difference of the cell is d.multidot..DELTA.n=0.50 .mu.m; the cell contains on both sides a linear polariser whose absorption axes are rotated through 90.degree. relative to one another, the front polariser and the preferential direction of the molecules forming an angle of 90.degree. at the substrates. Analogous results are obtained if the polarisers are arranged parallel to the preferential direction of the molecules. In the case of polarisers rotated through 90.degree. relative to one another, the cell becomes dark when addressed (known as positive contrast mode). Conversely, cells having polarisers arranged parallel to one another are dark in the unaddressed state and become bright when addressed (known as negative contrast mode).
The definition of the viewing angle .theta. is shown in FIG. 2. If the display is viewed obliquely from above, .theta. is &lt;0, and correspondingly for observation obliquely from below, .theta. is &gt;0. Characterisation of the viewing-angle dependence requires a second angle .PHI., which is in the plane perpendicular to .theta.=0.degree. and varies from 0-360.degree.; .PHI. is referred to below as the observation angle.
FIG. 1 shows-the relative contrast of the above cell as a function of the applied voltage for various viewing angles .theta.. A relative contrast of 100% is obtained if the transmission in the optical measuring device with the (halogen) lamp switched on and off is measured without a liquid-crystal cell. The position of the electro-optical characteristic line with respect to the voltage axis is described using voltage values V.sub.x,y,z, where a contrast of x% is observed at a viewing angle .theta. of y.degree. and a temperature of z.degree. C. V.sub.10,0,20 is frequently also known as the threshold voltage and V.sub.90,0,20 as the saturation voltage.
It can be seen from FIG. 1 that the shape and position of the electro-optical characteristic line with respect to the voltage axis change as the viewing angle .theta. is varied. If the display is arranged, for example, so that the best contrast is obtained for observation obliquely from below, for example for .theta..apprxeq.10.degree., and then the viewing angle .theta. is varied at a fixed voltage, for example at V.sub.90,0,20, a decrease in the relative contrast C is observed, i.e. the display becomes brighter, for .theta.&lt;0, i.e. for observation obliquely from above. For .theta.&gt;0, i.e. for observation obliquely from below, an increase in the contrast C is first obtained, i.e. the grey tone defined by V.sub.90,0,20 becomes darker. If the viewing angle is increased further, the grey tone becomes brighter again and the contrast C drops below the contrast observed in the case of small viewing angles .theta.. This phenomenon, known as inverse contrast, proves to be very disturbing during observation of a display. When a black/white display is observed obliquely from below (.theta.&gt;0.degree.), an increasing viewing angle (typically from about .theta..apprxeq.30.degree.) causes areas in the display which are dark grey for a relatively small positive .theta. to become black, whereas areas which are black for a relatively small positive .theta. appear dark grey at a large positive .theta.. In the case of colour displays, the effect causes some colours to be exchanged. A graphic measure of the strength of the inverse contrast is the extent of the fall in the electro-optical characteristic line at large positive viewing angles .theta. (for example .theta.=45.degree.) after having reached the maximum contrast for the first time (in FIG. 1 approximately the region of the electro-optical characteristic line between 1.8 and 2.7 V for .theta.=45.degree.).
In order to display grey shades, the cell is addressed in the region of the rising characteristic line, and the relative contrast values C, observed, for example, at V.sub.50,0,20, V.sub.60,0,20, V.sub.90,0,20, etc., correspond to different grey shades. In order to elucidate the phenomenon of inverse contrast, FIG. 3 shows this from the contrast values C measured at 2 different voltages V.sub.90,10,20 .+-.100 mV, in an isocontrast diagram as a function of the viewing angle .theta. and the observation angle .PHI.. If the quotient a=C (V.sub.90,10,RT +100 mV)/C (V.sub.90,10,RT -100 mV) is &gt;1, an increase in contrast is observed when the voltage is increased, and grey tones become correspondingly darker (normal contrast). If, by contrast, this quotient is less than 1, the contrast drops with increasing voltage and the grey tones become brighter (inverse contrast). In FIG. 3, the areas where 0.9&lt;a.ltoreq.1 are marked in pale grey, those where 0.8&lt;a.ltoreq.0.9 are marked in dark grey and those where a.ltoreq.0.8 are marked in black. It can be seen that a large region with inverse contrast arises for a conventional TN cell if the cell is observed obliquely from below.